1. Field of the Invention
The present invention is directed to a steam purity monitor which detects the presence of sodium hydroxide and sodium chloride in a steam turbine and, more particularly, to a device which monitors conductivity, temperature, and pressure in a steam turbine and uses a computer to indicate the presence of sodium hydroxide or sodium chloride based on the conductivity, temperature and pressure in the steam turbine.
2. Description of the Related Art
In a steam turbine, it is essential for the steam therein to remain free of chemical contaminants which cause corrosion. Sodium hydroxide and sodium chloride are two such contaminants which can cause serious damage. The presence of these substances in a steam turbine, even in very small amounts, can result in corrosion and related effects, including pitting corrosion, corrosion fatigue and stress corrosion. Particularly, sodium chloride affects the blades in the turbine and sodium hydroxide affects the rotor body of the turbine, which is made from a different alloy.
Conventionally, potential contaminants are monitored by sampling the feedwater and steam of the power cycle. When the monitors suggest that sodium chloride or sodium hydroxide is in the steam delivered to the turbine, the choice is to shut the turbine down, improve the purification of the feedwater used to make the steam, or to risk corrosion damage to the turbine. Current monitors are not accurate enough to reliably indicate whether corrosive solutions are forming on the turbine. Therefore, there is considerable likelihood that a turbine will be operated with corrosive solutions present on it or that a turbine will be shut down when no corrosive solutions are actually present on it. Either of these errors is costly. The first represents corrosion damage to equipment with possible safety hazards. The second represents unnecessary economic penalty of lost generation. For these reasons, it is highly desirably to detect the presence of sodium hydroxide with certainty and differentiate it from sodium chloride. With this information, the operator can decide whether to continue to operate the turbine or to shut it down.
In addition, corrosion damage from sodium hydroxide is faster and more widespread in the turbine than corrosion damage from salts. If sodium hydroxide is present in the steam going to the turbine, acid could be added to neutralize it. Conventional monitors are inadequately reliable to determine how much acid to add. For this reason it is desirable to detect the neutralization of sodium hydroxide present in the turbine.
Although previous attempts have been made to detect contaminants in a general sense, no previous device or method is known by which sodium hydroxide is specifically detected quickly and accurately. A method for preventing corrosion in a steam turbine is disclosed in U.S. Pat. No. 4,386,498, but this method is primarily directed to detection of conductivity in the turbine to generally indicate the presence of contaminants such as sodium chloride. The method is incapable of individually differentiating between different contaminants such as sodium hydroxide and sodium chloride. Such a differentiation is very important because only the detection of sodium hydroxide merits the extreme measure of adding acid to the turbine or taking the turbine off line and opening to clean it.